The rotational spectrum of the CHClF 2 –H 2 O weakly bound dimer has been measured using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5–18GHz range. The structure of the complex has been determined by analysis of the moments of inertia of five isotopologues of the dimer. The primary interaction between the two monomers is a weak C–H⋯O contact (R H⋯O =2.332(3) Å) with a C–Cl⋯H–O contact also present (R Cl⋯H =2.749(13) Å). The observed structure is in reasonable agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level, although these predict a Cl⋯H distance that is significantly longer than the experimental results indicate. The rotational transitions of all isotopologues containing H 2 O or D 2 O were doubled, with relative intensities of the observed transitions consistent with an internal rotation of the water molecule leading to exchange of equivalent hydrogen atoms. Fitting the upper and lower components of the transitions using an effective Hamiltonian with the ERHAM program has yielded an energy difference between the tunneling states of 16.0(4) GHz, resulting in an estimate of the barrier to internal rotation of 195(5) cm −1 (to be compared with an ab initio estimate of ∼117cm −1 ). The binding energy of the complex is estimated to be ∼5.5(2) kJ/mol (∼460cm −1 ) from a pseudo-diatomic approximation and assumption of a Lennard–Jones intermolecular potential.